In the United States and around the world, the demand for electrical power continues to grow. At the same time, aging transmission and distribution systems remain subject to occasional failures. Massive failures covering wide geographical areas and affecting millions of people have occurred, even in the United States which has historically enjoyed a relatively robust electrical power system. These problems with the capacity and reliability of the public power grid have driven the development of distributed energy resources (DER), small independent power generation and storage systems which may be owned by, and located near, consumers of electrical power.
One motivating factor is that distributed energy resources can provide more reliable power in critical applications, as a backup to the primary electrical supply. For example, an interruption of power to a hospital can have life-threatening consequences. Similarly, when power to a factory is interrupted, the resulting losses, for example in productivity, wasted material in process that must be scrapped, and other costs to restart a production line, can be catastrophic. In situations like these, where the loss of electrical power can have serious consequences, the cost of implementing a distributed energy resource as a backup can be justified.
Reliability is not the only factor driving the development of distributed energy resources. Power from a distributed energy resource can, in some cases, be sold back to the main power grid. Geographically distributed sources of power, such as wind, solar, or hydroelectric power, may be too limited or intermittent to be used as the basis for a centralized power plant. By harnessing these types of geographically distributed sources using multiple distributed energy resources, these types of power sources can supplement or replace conventional power sources, such as fossil fuels, when the main power grid is available, and provide backup to their owners when the main power grid is unavailable.
In this context, distributed energy resources (DER) have emerged as a promising option to meet customers current and future demands for increasingly more reliable electric power. Power sources for DER systems, sometimes called “microsources,” range in size and capacity from a few kilowatts up to 10 MW, they may include a variety of technologies, both supply-side and demand-side, and they are typically located where the energy is used.
Generally speaking, distributed energy resources can harness two broad categories of electrical power sources: DC sources, such as fuel cells, photovoltaic cells, and battery storage; and high-frequency AC sources, such as microturbines and wind turbines. Both types of sources are typically used to provide an intermediate DC voltage, that may be produced directly by DC sources, and produced indirectly from AC sources, for example by rectification. In both types of sources, the intermediate DC voltage is subsequently converted to AC voltage or current at the required frequency, magnitude, and phase angle for use. In most cases, the conversion from the intermediate DC voltage to the usable AC voltage is performed by a voltage inverter that can rapidly control the magnitude and phase of its output voltage.
Distributed energy resources are usually designed to operate in one of two modes: (1) “isolation” or “island” mode, isolated from the main grid, and (2) normal “grid” mode, connected to the main grid. For large utility generators, methods have been developed to allow conventional synchronous generators to join and to separate from the main electrical power grid smoothly and efficiently when needed. Because of fundamental differences between distributed energy resources, such as inverter based microsources or small synchronous generators, and centralized energy resources, these existing methods are not suitable to allow distributed energy resources to smoothly and efficiently transition between island mode and grid mode as the distributed energy resources join and separate from the main power grid.
For example, the fundamental frequency in an inverter is typically derived from an internal clock that does not change as the system is loaded. This arrangement is very different from that of synchronous generators typically used in centralized power systems, in which the inertia from spinning mass determines and maintains system frequency. Inverter-based microsources, by contrast, are effectively inertia-less, so alternative methods must be used to maintain system frequency in an inverter-based microsource.
Another difference between distributed energy resources and centralized energy resources relates to communication and coordination. A centralized electrical power utility is in a position to monitor and coordinate the production and distribution of power from multiple generators. In contrast, distributed energy resources may include independent producers of power who have limited awareness or communication with each other. Even if the independent producers of power are able to communicate with each other, there may not be any effective way to ensure that they cooperate.
Thus, there is a need for methods of controlling microsources in distributed energy resources to ensure that these resources can connect to or isolate from the utility grid in a rapid and seamless fashion, that reactive and active power can be independently controlled, and that voltage sag and system imbalances can be corrected. Further, there is a need for control of the microsources, and in particular the inverters used to supply power to the grid, based solely on information available locally at the inverter so that no communication or coordination between microsources is necessary. Yet further, there is a need for a local controller at the microsource to enable “plug and play” operation of the microsource. In other words, there is a need to add microsources to a distributed energy resource system without changes to the control and protection of units that are already part of the system.